Ion chromatography (IC) is often used to test for organic and inorganic ions in various sample materials, including waste water, lake water, underground water, process streams, air samples, and food products. Generally, two ion exchange columns, often called a guard column and an analytical column, are used in series. An eluant, commonly referred to as the mobile phase, carries the sample to be tested through the ion exchange columns. The analytical column effluent is then allowed to go through a suppressor column and finally through an electrical conductivity detector. The analytical column separates the ions from the remainder of the injected sample, and the suppressor column suppresses the conductance of the eluant, while promoting the conductance of the separated ions. Typically, the suppressor column is filled with an ion exchange resin that suppresses the conductivity of the eluant electrolyte. The analytical column and the suppressor column can be designed to test for either anions or cations. Usually, the eluant constitutes an electrolyte, such as dilute base or acid in deionized water.
The conventional IC system described above suffers from several drawbacks. Periodically, the suppressor column must be regenerated or replaced, because the ion exchange resin gradually loses its functioning ability. For instance, in the case of IC for anions, the reaction shown below eventually converts all of the ion-exchange sites in the suppressor column in the H.sup.+ form to the Na.sup.+ form, causing the suppressor column to lose its ability to capture Na.sup.+ from the eluant (a mixture of NaHCO.sub.3 and Na.sub.2 CO.sub.3):
NaHCO.sub.3 +Na.sub.2 CO.sub.3 +3strong resin-H.sup.+ .fwdarw.3strong resin-Na.sup.+ 2H.sub.2 CO.sub.3
As the reaction shown above proceeds, the background conductance of the eluant drifts upward and, at the same time, signal enhancement for the analyte is lowered, resulting in poor precision for determination of various ionic species. Additionally, in many applications of chromatographic analysis, a phenomenon referred to as a "water dip" occurs and is represented on the graphic output by a negative peak as compared to the positive peaks of the various ionic species. As the resin material in the suppressor column deteriorates, this negative peak shifts towards the first positive peak, e.g., fluoride, and can actually overlap the positive peak. This makes interpretation of the graphic output more difficult and adversely affects accuracy and precision. Also, it usually takes 8-10 hours to complete regeneration of an exhausted suppressor column, resulting in impairment to productivity.
Additionally, many existing suppressor columns have substantial internal dead space that leads to band broadening, rendering it difficult to accurately determine the ionic species within the samples being tested. Band broadening can be limited by using somewhat smaller suppressor columns with less dead space, but the industry has had limited success with the smaller suppressor columns because they require frequent regeneration. Thus, the testing of samples is constantly interrupted while the suppressor column is either replaced or regenerated. Attempts also have been made to reduce the frequency of regeneration by providing a relatively low capacity for exchange in the analytical column, but this is problematic because the analytical column capacity can be overloaded and resolution between the ionic species is reduced.
The determination of weak-acid or weak-base ions using a suppressor column is complicated by ion-exclusion effects in the unexhausted portion of the column. For instance, taking an example of a weak-acid anion such as nitrite, the peak height for a known NO.sub.2 concentration is smaller when the suppressor column is unexhausted than that when the suppressor column is exhausted. As the suppressor column becomes exhausted, the ion-exclusion effects decrease, resulting in increased peak height.
The present invention eliminates the various drawbacks of existing methods and devices for performing suppressed ion chromatography.